In the related art, surface-light-emitting optical fibers are collected in a bundle form, a sheet form or a string form and are used as a light source of an illumination device (see, e.g., Japanese Unexamined Patent Application Publication No. 1994-186428). The optical fibers are fixed to, for example, a building material of a ceiling, a wall or the like in a living space and can be used in illuminating and decorating the ceiling or the wall.
However, if the aforementioned optical fibers are directly installed in the ceiling or the wall, brightness unevenness may be generated in the ceiling or the wall because the ceiling or the wall existing near the optical fibers are locally illuminated by the light emitted from the optical fibers. Further, in general, the optical fibers have flexibility and, therefore, the optical fibers mounted to the ceiling or the wall tend to be bent by their own weight.